Solid state zirconia oxygen sensors are widely used for fuel-air stoichiometry measurements in exhaust gas streams and are ubiquitous in automotive gasoline engine control applications. These sensors also are known as lambda sensors, fuel air stoichiometry sensors or combustion control sensors. They measure the difference in potential between a platinum exhaust gas (equilibrium) electrode and a platinum air (reference) electrode to measure the thermodynamic oxygen concentration of the exhaust gas mixture. An exhaust gas contains oxidizable species such as hydrocarbons and/or carbon monoxide and oxygen gas. It has been shown that under low exhaust gas temperature conditions platinum (Pt) metal does not always catalyze automobile engine exhaust gases to equilibrium with oxygen in the exhaust stream. This results in an error in measurement in the fuel/air stoichiometry, especially under lean fuel conditions.
Oxidizable gas sensors detect oxidizable gases such as hydrocarbons, hydrogen, carbon monoxide in gas streams containing oxygen, and the like. Such sensors typically possess a nonequilibrium electrode with a very strong non-equilibrium response electrical potential and an equilibrium counter electrode that measures the equilibrium electrical potential from the gas mixture. Both of these electrodes are exposed to the analyte gas stream. A third reference electrode that is not exposed to the exhaust gas stream and that is exposed to a fixed oxygen partial pressure may also be present. This third electrode enables the simultaneous measurement of the equilibrium oxygen pressure of the gas mixture. Materials for the non-equilibrium and equilibrium electrodes for these sensors have been selected from various dissimilar materials:
1. two electrodes of dissimilar metals, e.g., platinum and gold; PA1 2. a metal oxide mixed conductor electrode and a metal electrode, e.g., lanthanum cobalt oxide and gold; PA1 3. two electrodes of different mixed conducting oxides, e.g., lanthanum cobalt oxide, lanthanum manganese oxide, and the like.
Sensors of the second and third type have been described previously (patent application Ser. No. 08/640,451, filed Apr. 30, 1996, Solid State Gas Sensor, incorporated herein by reference). These sensors have two electrodes with dissimilar electrocatalytic properties for the electro-oxidation of a gaseous species and the electrochemical reduction of oxygen. A mixed electrochemical potential is established on the dissimilar electrodes due to differences in electrokinetic rates for the gas-ion redox reactions. A counter electrode develops a potential that is closer in value to the Nernstian equilibrium oxygen potential for the mixture than the working electrode. The difference between the mixed potentials established on the electrodes is the sensor output signal.
Sensors for measuring oxidizable gas concentrations in exhaust gas streams should have a working electrode with a very strong nonequilibrium response and an equilibrium electrode that measures the equilibrium potential of the gas mixture. In the combustion gas application, the sensors typically run colder than zirconia oxygen sensors and effective catalysis at the equilibrium electrode is an important requirement over a working temperature range of interest.
Various objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.